x86/asm/entry/32: Simplify pushes of zeroed pt_regs->REGs

Use of a temporary R8 register here seems to be unnecessary.

"push %r8" is a two-byte insn (it needs REX prefix to specify R8),
"push $0" is two-byte too. It seems just using the latter would be
no worse.

Thus, code had an unnecessary "xorq %r8,%r8" insn.
It probably costs nothing in execution time here since we are probably
limited by store bandwidth at this point, but still.

Run-tested under QEMU: 32-bit calls still work:

 / # ./test_syscall_vdso32
 [RUN]	Executing 6-argument 32-bit syscall via VDSO
 [OK]	Arguments are preserved across syscall
 [NOTE]	R11 has changed:0000000000200ed7 - assuming clobbered by SYSRET insn
 [OK]	R8..R15 did not leak kernel data
 [RUN]	Executing 6-argument 32-bit syscall via INT 80
 [OK]	Arguments are preserved across syscall
 [OK]	R8..R15 did not leak kernel data
 [RUN]	Running tests under ptrace
 [RUN]	Executing 6-argument 32-bit syscall via VDSO
 [OK]	Arguments are preserved across syscall
 [NOTE]	R11 has changed:0000000000200ed7 - assuming clobbered by SYSRET insn
 [OK]	R8..R15 did not leak kernel data
 [RUN]	Executing 6-argument 32-bit syscall via INT 80
 [OK]	Arguments are preserved across syscall
 [OK]	R8..R15 did not leak kernel data

Signed-off-by: Denys Vlasenko <dvlasenk@redhat.com>
Acked-by: Andy Lutomirski <luto@kernel.org>
Cc: Andy Lutomirski <luto@amacapital.net>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Brian Gerst <brgerst@gmail.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Kees Cook <keescook@chromium.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Will Drewry <wad@chromium.org>
Cc: linux-kernel@vger.kernel.org
Link: http://lkml.kernel.org/r/1462201010-16846-1-git-send-email-dvlasenk@redhat.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>

arch/x86/entry/entry_64_compat.S

@@ -72,24 +72,23 @@ ENTRY(entry_SYSENTER_compat)
 	pushfq				/* pt_regs->flags (except IF = 0) */
 	orl	$X86_EFLAGS_IF, (%rsp)	/* Fix saved flags */
 	pushq	$__USER32_CS		/* pt_regs->cs */
-	xorq    %r8,%r8
-	pushq	%r8			/* pt_regs->ip = 0 (placeholder) */
+	pushq	$0			/* pt_regs->ip = 0 (placeholder) */
 	pushq	%rax			/* pt_regs->orig_ax */
 	pushq	%rdi			/* pt_regs->di */
 	pushq	%rsi			/* pt_regs->si */
 	pushq	%rdx			/* pt_regs->dx */
 	pushq	%rcx			/* pt_regs->cx */
 	pushq	$-ENOSYS		/* pt_regs->ax */
-	pushq   %r8                     /* pt_regs->r8  = 0 */
-	pushq   %r8                     /* pt_regs->r9  = 0 */
-	pushq   %r8                     /* pt_regs->r10 = 0 */
-	pushq   %r8                     /* pt_regs->r11 = 0 */
+	pushq   $0			/* pt_regs->r8  = 0 */
+	pushq   $0			/* pt_regs->r9  = 0 */
+	pushq   $0			/* pt_regs->r10 = 0 */
+	pushq   $0			/* pt_regs->r11 = 0 */
 	pushq   %rbx                    /* pt_regs->rbx */
 	pushq   %rbp                    /* pt_regs->rbp (will be overwritten) */
-	pushq   %r8                     /* pt_regs->r12 = 0 */
-	pushq   %r8                     /* pt_regs->r13 = 0 */
-	pushq   %r8                     /* pt_regs->r14 = 0 */
-	pushq   %r8                     /* pt_regs->r15 = 0 */
+	pushq   $0			/* pt_regs->r12 = 0 */
+	pushq   $0			/* pt_regs->r13 = 0 */
+	pushq   $0			/* pt_regs->r14 = 0 */
+	pushq   $0			/* pt_regs->r15 = 0 */
 	cld
 
 	/*
@@ -205,17 +204,16 @@ ENTRY(entry_SYSCALL_compat)
 	pushq	%rdx			/* pt_regs->dx */
 	pushq	%rbp			/* pt_regs->cx (stashed in bp) */
 	pushq	$-ENOSYS		/* pt_regs->ax */
-	xorq    %r8,%r8
-	pushq   %r8                     /* pt_regs->r8  = 0 */
-	pushq   %r8                     /* pt_regs->r9  = 0 */
-	pushq   %r8                     /* pt_regs->r10 = 0 */
-	pushq   %r8                     /* pt_regs->r11 = 0 */
+	pushq   $0			/* pt_regs->r8  = 0 */
+	pushq   $0			/* pt_regs->r9  = 0 */
+	pushq   $0			/* pt_regs->r10 = 0 */
+	pushq   $0			/* pt_regs->r11 = 0 */
 	pushq   %rbx                    /* pt_regs->rbx */
 	pushq   %rbp                    /* pt_regs->rbp (will be overwritten) */
-	pushq   %r8                     /* pt_regs->r12 = 0 */
-	pushq   %r8                     /* pt_regs->r13 = 0 */
-	pushq   %r8                     /* pt_regs->r14 = 0 */
-	pushq   %r8                     /* pt_regs->r15 = 0 */
+	pushq   $0			/* pt_regs->r12 = 0 */
+	pushq   $0			/* pt_regs->r13 = 0 */
+	pushq   $0			/* pt_regs->r14 = 0 */
+	pushq   $0			/* pt_regs->r15 = 0 */
 
 	/*
 	 * User mode is traced as though IRQs are on, and SYSENTER
@@ -316,11 +314,10 @@ ENTRY(entry_INT80_compat)
 	pushq	%rdx			/* pt_regs->dx */
 	pushq	%rcx			/* pt_regs->cx */
 	pushq	$-ENOSYS		/* pt_regs->ax */
-	xorq    %r8,%r8
-	pushq   %r8                     /* pt_regs->r8  = 0 */
-	pushq   %r8                     /* pt_regs->r9  = 0 */
-	pushq   %r8                     /* pt_regs->r10 = 0 */
-	pushq   %r8                     /* pt_regs->r11 = 0 */
+	pushq   $0			/* pt_regs->r8  = 0 */
+	pushq   $0			/* pt_regs->r9  = 0 */
+	pushq   $0			/* pt_regs->r10 = 0 */
+	pushq   $0			/* pt_regs->r11 = 0 */
 	pushq   %rbx                    /* pt_regs->rbx */
 	pushq   %rbp                    /* pt_regs->rbp */
 	pushq   %r12                    /* pt_regs->r12 */